Folic acid capping Bi3+-doped Ag quantum dots for enzyme-like dual-mode recognition of toxic S2− and visual sensing of NO2−

Background NO 2 − and S 2− are two kinds of common toxic anions widely distributed in environmental water, soil and food products. Human beings have suffered a lot of diseases from intake of excessive NO 2 − or S 2− , i.e ., infantile methemoglobin , cancer and even to death. Although tremendous efforts have been afforded to monitor NO 2 − and S 2− , most were high instrument-depended with complex processing procedures. To keep food safety and to protect human health, it will be a huge challenge to develop a convenient and efficient way to monitor S 2− and NO 2 − in practice. Results A kind of folic acid capping Bi 3+ -doped Ag quantum dots (FA@Bi 3+ -Ag QDs) was developed for the first time by one-pot homogeneous reduced self-assembly. Not only did FA@Bi 3+ -Ag QDs possess intrinsic fluorescent property, it expressed synergistic peroxidase-like activity to catalyze the redox of 3,3′,5,5′-tetramethylbenzidine (TMB) and H 2 O 2 with K m / v max of 0.087 mM/6.61 × 10 −8  M s −1 and 6.42 mM/6.25 × 10 −7  M s −1 respectively. Interestingly, trace S 2− could exclusively alter its fluorescent property and peroxidase-like activity, exhibiting significant hypochromic and “turn-on” fluorescent effects. While trace NO 2 − could make FA@Bi 3+ -Ag QDs-TMB-H 2 O 2 system hyperchromic. Under the optimized conditions, FA@Bi 3+ -Ag QDs were applied for dual-mode recognition of S 2− and visual sensing of NO 2 − in real food samples with satisfactory recoveries, i.e ., 100.7–107.9 %/95.8–104.7 % and 97.2–104.8 % respectively. The synergistic enzyme-mimic mechanism of FA@Bi 3+ -Ag QDs and its selective response mechanisms to S 2− and NO 2 − were also proposed. Significance This represents the first nanozyme-based FA@Bi 3+ -Ag QDs system for dual-mode recognition of S 2− and visual sensing of NO 2 − , well meeting the basic requirement in drinking water set by WHO. It will offer a promising way for multi-mode monitoring of different pollution using the same nanozyme-based sensor.